Rheometers are used to determine the flow characteristics and visco-elastic properties of materials such as liquids, structured liquids and liquefiable materials. Rheometers typically measure material behavior such as yield stress, kinetic properties, complex viscosity, modulus, creep, and recovery. In general, rheometric measurements are made by rotating, deflecting or oscillating a measuring object in a material and measuring, for example, the torque (i.e. shear stress σ) required to do so. Typically, a rheometer comprises at least two bounding surfaces, one or more of which is moveable by rotational or other means. In a standard setup the bounding surface comprising a top flat plate, a bottom cone or vice versa and alternately a means of containing the material around a perimeter of the plate/cone.
A sample is then positioned between these surfaces, and the movable portion, typically the cone, is subject to a variable shear stress or speed. Shear rate {dot over (γ)} is measured as a function of applied shear stress σ to determine flow characteristics of the sample, one such standard measurement being viscosity. The material itself being characterized by a series of measurements at the beginning, during application of and at the end of the applied stress. At present visual representations of the material undergoing change lack clarity and depth of field.
Structured fluids such as foams, colloids, micelles, granular materials and polymers display intricate dynamic behavior that lack an adequately complete description. The flow characteristics of structured fluids can be described more accurately and/or in greater detail through direct visualization devices and related methods. However, the state of the art does not include simultaneous rheological measurements in conjunction with direct visualization and determination of the velocity field within the sample fluid. Thus, the art lacks devices and methods for direct visualization of velocity fields within rheological samples and lacks methods for visualizing the morphology of sheared samples along the velocity gradient direction.